How can I add more axes of symmetry?
Another way is to use the "object offset" option in the Array modifier.


Create your pillar/section of the cupola
Apply Rotation &amp; Scale (or move the origin of the cupola to the origin of your scene)





Create an Empty at the origin





Add an Array modifier to your cupola section, select the "Object Offset" checkbox, and choose the Empty





Enter the number of times to repeat your object (in this case I chose 10)





Select the Empty and rotate it 360 degrees divided by the number of items.  In my example, since I want 10 copies, I rotate the empty by 36 degrees (360/10).  If you wanted a half-circle, you could use 180. 




If you get weird offsets, make sure the object you want to repeat has no rotation or scale on it.  Then, you can control rotation, scale, and offset via the Empty.